LG LST-5100P set top box. Several times a d...

Bought LG LST-5100P set top box recently as a second hand. It has a very strange fault - "frozen picture" with sound going. Once it happens - all buttons on the front panel are not working at all as well as a remote control. It is not even possible to turn it off using remote control or power button on the front panel. I have to disconnect STB from electric socket and turn it on. This is the only way to start it again. I have got the "frozen picture" several times through evening watching. Sometimes this happens 2-3 times through the long watch, sometimes I have it every 10-15 minutes. Espesially on Channel 7. I've tested STB through the last week every day on two TVs : new LCD TV and older model CRT 29" TV. The result was the same - "frozen picture"(just a frame) after while with a sound going and no functions from the front panel buttons and remote control. I even antenna amplifier and attenuator antenna to get better signal but haven't got any change. What kind of problem is that? And how much is to get it fixed roughly? Thanks